the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jul 2025
Measurement report: Observational insights into the impact of dust transport on atmospheric dicarboxylic acids in ground region and free troposphere
Abstract. This study investigated the vertical distribution of PM2.5 and size-segregated aerosols at the foot and top of Mount Hua in Northwest China, focusing on C2 formation and its δ13C characteristics influenced by dust transport. Under non-dust conditions, PM2.5 and diacids concentrations at the foot were 4.5 and 2.1 times higher than those at the top, with stronger local anthropogenic signals (C9, 5.67 times higher) and pronounced diurnal diacids variation. Higher C2/C4 (5.84 vs. 4.74), C3/C4 (1.04 vs. 0.56) ratios and δ13C values (-21.5 ‰ vs. -27.6 ‰) at the top indicated photochemical aging during aerosol transport. C2 concentration was positively correlated with aerosol liquid water content and its size distribution pattern matched with precursors, confirming aqueous-phase oxidation as the dominant formation pathway. During dust events, PM2.5 concentrations at the foot and top reached 457 μg m−3 and 165 μg m−3, but C2 concentrations in PM2.5 decreased by 59 % (foot) and 25 % (top), while δ13C values of C2 exhibited a significant positive shift (foot: -27.6 ‰ to -23.9 ‰; top: -21.5 ‰ to -13.2 ‰), attributed to alkaline dust catalyzing 13C-enriched oxalate formation. mGly became the second most abundant acid due to enrichment on dust-particle surfaces. Size-segregated data revealed decreased fine-particle C2 but increased coarse-particle C2, elevating the coarse-to-fine ratio from 0.3~0.4 to 0.6~1.1. These findings offer valuable insights into the altitude-dependent transformation of SOA affected by dust transport, enhancing our understanding of mountain atmospheric chemistry and regional air quality.
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Status: open (until 10 Sep 2025)
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RC1: 'Comment on egusphere-2025-3094', Anonymous Referee #1, 07 Aug 2025
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3094/egusphere-2025-3094-RC1-supplement.pdf
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RC2: 'Comment on egusphere-2025-3094', Anonymous Referee #2, 02 Sep 2025
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The manuscript examines vertical gradients of PM2.5 and size-segregated aerosols at the foot and summit of Mount Hua, emphasizing oxalic acid (C₂) formation and δ¹³C signatures under dust events. The dataset has clear value and the topic—dust impacts on diacids and SOA pathways across altitude—is important. However, there are substantial questions and the English requires substantial polishing.
Specific comments
- 88–91: Wang et al. (2015) discovered that nitric acid and nitrogen oxides react with dust to form calcium nitrate, which absorbs water vapor and creates a liquid phase on the dust surface. This process promotes the oxidation of water-soluble organic precursors to C2, with nitrate playing a crucial role.
You cite nitrate’s role but omit critical evidence that water-containing nitrate coatings (Ca(NO3)2) facilitate aqueous-phase SOA formation due to their very low deliquescence RH (Ca(NO3)2 absorbs water under atmospheric RH > 8%). Li et al. (National Science Review, 12, 10.1093/nsr/nwaf221, 2025) provide direct evidence for this mechanism; that work explicitly discussed where nitrate coatings are invoked.
- Statistical support for “most striking differences” throughout your manuscript.
Wherever you describe significant differences (e.g., foot vs. top; dust vs. non-dust; day vs. night), please perform and report statistical tests (e.g., two-sample t-tests with assumptions stated). The manuscript currently lacks the statistical support needed to substantiate claims of significance.
- Introduction section — Scientific motivation for foot vs. top comparison
You reference prior work (Shen et al., 2023) that already found distinct C₂ formation pathways at different elevations of Mount Hua during the summer. Please state explicitly: (i) the knowledge gap left by your previous studies, (ii) what new question or hypothesis this study addresses beyond the prior work.
- 173–179 — Interpretation of diurnal behavior at altitude
The statement that oxalic acid (C2) and its precursors show no significant diurnal fluctuation at the summit, implying limited local photochemical contributions aloft, is presented in the manuscript without supporting references or mechanistic discussion. This interpretation seems to unclear and potentially inconsistent with established understanding. Cloud and in-cloud processes are widely recognized as important pathways for aqSOA formation in the free troposphere, and oxalate is known to associate preferentially with Fe-containing particles during in-cloud processing (e.g., Zhang et al., 2019, DOI: 10.1021/acs.est.8b05280). Fe is also known to catalyze the photochemical decomposition of oxalic acid. In this context, the claim that high-altitude C₂ shows no significant diurnal variation requires further justification and supporting references, as it appears difficult to reconcile with existing literature.
- 241: You attribute higher OC fraction to “enhanced photochemical reactions and gas-particle conversion,” but no direct evidence is shown.
Earlier text in your manuscript also suggests mixing with anthropogenic carbonaceous aerosols during long-range transport, and you report elevated azelaic acid (biomass burning tracer) at the summit. The manuscript does not provide clear evidence to distinguish between secondary formation and primary biomass-burning/transport contributions as explanations for the higher OC fraction.
- 246: You ascribe higher SO42- mainly to heterogeneous reactions on dust particle surfaces.
An alternative pathway—direct contribution from dust-borne sulfate species such as CaSO4 —receives insufficient consideration in the text. The current discussion does not adequately address this alternative or reconcile it with the observations.
- 307–310 “The C5 at the foot likely mainly comes from local emission, such as the oxidation of cyclopentene in vehicle exhaust, whereas the high-altitude area is influenced by a combination of long-range transport and local processes The text does not present the necessary observational support and data analysis to substantiate this source attribution.
- 332–337 — Coarse vs. fine C2 and the nitrate-coating hypothesis
The manuscript reports a shift from fine- to coarse-mode C2 during dust events and links this to nitrate coatings formed via CaCO3 + HNO3 → Ca(NO3)2. However, size-resolved evidence for Ca2+ and NO3- is not presented in the manuscript, so the causal link to a water-containing nitrate coating is not demonstrated. Relevant literature (e.g., W. J. Li & L. Y. Shao, ACP, 2009; Zhi et al., ES&T, 2025) that discusses the role of Ca-rich coatings during dust aging should be integrated into the discussion.
- English polishing and wording precision
- 49: “secondary substances” → “secondary aerosols”.
- 75: “photocatalytic reactions” → “photochemical reactions”.
- 163: “The contribution ratios of C9 at the two sites were 8.5% and 3.2%, respectively” → Specify “contribution to what?” (e.g., total diacids, total organic acids, or OC). Define the denominator.
- 233: Avoid “modest increase.” Provide exact statistics and confidence intervals; define “modest” quantitatively.
- 255: “a decrease of 59%” → “decrease by 59%”.
- 305: “single peak” → “unimodal distribution”.
- 331: “altered the particle size distribution characteristics” → “impacted the size distributions of diacids in aerosols”.
Citation: https://doi.org/10.5194/egusphere-2025-3094-RC2
Data sets
Measurement reports: Observational Insights into the Impact of Dust Transport on Atmospheric Dicarboxylic Acids in Ground region and Free Troposphere [Data set] L. Jianjun and S. Minxia https://zenodo.org/doi/10.5281/zenodo.15788834
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